Probing the Low-x Structure of the Nucleus with the PHENIX Detector

One of the fundamental goals of the PHENIX experiment is to understand the structure of cold nuclear matter, since this serves as the initial state for heavy-ion collisions. Knowing the initial state is vital for interpreting measurements from heavy-ion collisions. Moreover, the structure of the cold nucleus by itself is interesting since it is a test-bed for our understanding of QCD. In particular there is the possibility of novel QCD effects such as gluon saturation at low-x in the nucleus. At RHIC we can probe the behavior of gluons at low-x by measuring the pair cross-section of di-hadrons from di-jets in d+Au collisions. Our results show a systematic decrease in the pair cross-section as one goes to smaller impact parameters of the nucleus, and also as one goes to lower Bjorken x. There is a possibility that these interesting effects come from gluon recombination at low x in the Au nucleus.Comment: 4 pages, 3 figures, to be published in the Proceedings of the XX International Workshop on Deep Inelastic Scattering, University of Bonn, 26-30th March 201

Kinesisk perspektiv på ytringsfrihet og Internettkontroll : En analyse av kinesiske akademiske artikler

Denne oppgaven forsøker å avklare hvorfor Kina har et annet perspektiv på Internettkontroll enn de fleste vestlige land. Til tross for at mye av informasjonen på Internett blir overvåket, filtrert og sensurert av statlige organer, aksepterer de fleste kineserne myndighetenes kontrollerende tilstedeværelse på Internett. Når vestlige land kritiserer den kinesiske Internettkontrollen uten at kineserne bryr seg i særlig grad, ser jeg behov for å forstå bakgrunnen for denne aksepten. Undersøkelsen jeg har utført, er basert på en dokumentanalyse av en rekke kinesiske akademiske artikler om ytringsfrihet på Internett. Jeg har satt meg inn i akademikernes syn på ytringsfrihet – hva de mener kjennetegner informasjon og kommunikasjon på Internett – hvilke problemer de forbinder med Internett uten statlig regulering – og hvordan de mener disse problemene bør håndteres. Det kinesiske synet på ytringsfrihetens begrunnelser er svært likt det vi i Norge kaller for sannhetsprinsippet, autonomiprinsippet og demokratiprinsippet. Likevel viser mange av akademikernes argumenter at ytringsfrihet på Internett i mange tilfeller ikke tjener "sannhet, fri meningsdannelse og demokrati". Ytringsfrihet på Internett forbindes med informasjonsmangfold, som ikke nødvendigvis er positivt. Deler av denne informasjonen kan svekke den ideologiske og politiske standarden hos det kinesiske folk, særlig unge studenter som er i en kritisk fase med tanke på å etablere et ideologisk og politisk sterkt fundament. Kinas kultur kan bli utvannet og den nasjonalistiske stolthetsfølelsen hos folket kan forsvinne. Derfor er det viktig at Kina ikke bare åpner armene sine for Internetteknologien, men den må også tilpasses det kinesiske samfunnet. Tradisjonelt sett har myndighetene i Kina hatt en slags oppdragerrolle overfor folket sitt. Derfor er det å styrke og tilpasse det ideologiske og politiske oppdragelsesarbeidet et av de viktigste tiltakene i møtet med Internettalderen. Selv om de funnene denne oppgaven har kommet frem til, ikke nødvendigvis kan brukes til å forsvare den kritikkverdige Internettkontrollen som eksisterer i Kina i dag, gir den et unikt innblikk i kinesernes perspektiv på ytringsfrihet og Internettkontroll

Nonlinear force-free models for the solar corona I. Two active regions with very different structure

With the development of new instrumentation providing measurements of solar photospheric vector magnetic fields, we need to develop our understanding of the effects of current density on coronal magnetic field configurations. The object is to understand the diverse and complex nature of coronal magnetic fields in active regions using a nonlinear force-free model. From the observed photospheric magnetic field we derive the photospheric current density for two active regions: one is a decaying active region with strong currents (AR8151), and the other is a newly emerged active region with weak currents (AR8210). We compare the three-dimensional structure of the magnetic fields for both active region when they are assumed to be either potential or nonlinear force-free. The latter is computed using a Grad-Rubin vector-potential-like numerical scheme. A quantitative comparison is performed in terms of the geometry, the connectivity of field lines, the magnetic energy and the magnetic helicity content. For the old decaying active region the connectivity and geometry of the nonlinear force-free model include strong twist and strong shear and are very different from the potential model. The twisted flux bundles store magnetic energy and magnetic helicity high in the corona (about 50 Mm). The newly emerged active region has a complex topology and the departure from a potential field is small, but the excess magnetic energy is stored in the low corona and is enough to trigger powerful flares.Comment: 11 pages, 11 figure

Production of Photons and Dileptons in the Glasma

We study the production of photons and dileptons during the pre-equilibrium Glasma stage in heavy ion collisions and discuss the implications in light of the PHENIX data. We find that the measured distributions of such electromagnetic emissions, while having some features not well understood if hypothesized to entirely arise from a thermalized Quark-Gluon Plasma, have some qualitative features that might be described after including effects from a thermalizing Glasma. The shape and centrality dependence of the transverse momentum spectra of the so-called "thermal photons" are well described. The mass and transverse momentum dependence of intermediate mass dileptons also agree with our estimates. The low transverse momenta from which the excessive dileptons (in low to intermediate mass region) arise is suggestive of emissions from a Bose condensate. We also predict the centrality dependence of dilepton production. Uncertainties in the current approach and improvements in the future are discussed.Comment: 24 pages, 7 figures, new results and references added in v

Nonlinear force-free modeling of the solar coronal magnetic field

The coronal magnetic field is an important quantity because the magnetic field dominates the structure of the solar corona. Unfortunately direct measurements of coronal magnetic fields are usually not available. The photospheric magnetic field is measured routinely with vector magnetographs. These photospheric measurements are extrapolated into the solar corona. The extrapolated coronal magnetic field depends on assumptions regarding the coronal plasma, e.g. force-freeness. Force-free means that all non-magnetic forces like pressure gradients and gravity are neglected. This approach is well justified in the solar corona due to the low plasma beta. One has to take care, however, about ambiguities, noise and non-magnetic forces in the photosphere, where the magnetic field vector is measured. Here we review different numerical methods for a nonlinear force-free coronal magnetic field extrapolation: Grad-Rubin codes, upward integration method, MHD-relaxation, optimization and the boundary element approach. We briefly discuss the main features of the different methods and concentrate mainly on recently developed new codes.Comment: 33 pages, 3 figures, Review articl